import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode() {}
      TreeNode(int val) { this.val = val; }
      TreeNode(int val, TreeNode left, TreeNode right) {
          this.val = val;
          this.left = left;
          this.right = right;
      }
  }
public class Test {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList();
        if(root == null) return ret;
        Queue<TreeNode> queue = new LinkedList();
        queue.offer(root);
        while(!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> tem = new ArrayList<>();
            while(size != 0) {
                TreeNode cur = queue.poll();
                tem.add(cur.val);
                size --;
                if(cur.left != null) {
                    queue.offer(cur.left);
                }
                if(cur.right != null) {
                    queue.offer(cur.right);
                }
            }
            ret.add(tem);
        }
        return ret;
    }
}

